AbstractIn the early 1980’s Johnson & Johnson developed a revolu-

tionary process for the production of webs containing superab-sorbent polymers (SAP) produced by in-situ polymerization ofpartially neutralized acrylic monomers directly on a syntheticnonwoven substrate [1, 2, 3, 4]. A fresh look at this forgottentechnology will be presented from both a manufacturing andapplication perspective. In particular, In-situ, SAP-containingnonwovens offer many unique properties for application in thepersonal hygiene industry, such as, improved fluid acquisition,permeability, compressibility and pH control. These materialsalso provide a more homogenous SAP distribution, eliminate theneed for SAP powder handling and have superior wet integrityas compared to conventional fluff pulp/SAP air laid structures.This technology also offers some unique opportunities fordesigning and manufacturing profiled absorbent articles withspecific zones tailored to perform specific functions.

IntroductionTypically, conventional superabsorbent powder polymer pro-

duction begins with solution polymerization of partially neutral-ized acrylic acid along with a small amount of a crosslinkingagent in water. The polymerization results in a water insoluble,water swellable gel containing approximately 25 to 40 % solids,which must then be cut, dried, milled and sifted to produce apowdered SAP product with a typical particle size rangingbetween 100 to 800 µm. The sifting operation typically generatesa fines stream that must be recycled back into the productionprocess creating a production bottleneck. The finished SAPproduct is then shipped to a hygiene-industry converter where itis blended with fibrillated wood fluff to form the absorbent corestructure of a personal hygiene article such as a diaper.

In an in-situ SAP process, the partially neutralized acrylic acidmonomer solution is applied directly to a nonwoven substrateand polymerized. The web may be fed to the process either as apre-manufactured roll good or, preferably, made in-line from

bulk staple fiber using a carding operation. The monomer solu-tion may be applied to the web using a variety of applicationtechniques such as brush coating, pressurized liquid spray, air-assisted spray or airless spray. Initiation of the polymerizationmay be carried out by using a redox package, thermal initiation,UV, electron beam radiation or a combination of methods.Partial drying of the web may be achieved using the heat of poly-merization if the acrylic acid concentration of the monomer solu-tion is above 30%. The moisture content of the final product maybe adjusted to the desired level using either a through-air, airflotation, or forced-air infrared dryer. The dried product is thenmechanically softened, slit to the desired use width and wound.A schematic comparison of a conventional SAP productionprocess and an in-situ process is shown in Figure 1.

The in-situ SAP process offers a number of potential advan-tages for hygiene converters over conventional technology. Sincethe process produces stable, immobilized SAP structures, it elim-inates the need for SAP powder handling and the associateddust exposure issues. It also offers the potential to provide moreuniform SAP distribution in absorbent core structures and willremain stable during transport. The immobilized SAP particlesalso remain stable even in the hydrated state. Other possibilitiesinclude zoned SAP gradients and strategic placement of SAP’swith different performance properties within the core structure.The technology further offers the possibility of reducing produc-tion costs through the minimization of SAP processing steps.

Nonwoven SubstrateOne of the major objectives of the in-situ process is the forma-

tion and bonding of discrete SAP micro-droplets to individualweb fibers as shown in Figure 2. An optimized nonwoven struc-ture is critical to the achievement of this state. Typically, a high-loft nonwoven structure is required, depending on the desiredSAP loading and application. Key substrate properties includethe basis weight, bulk density, loft and porosity. A delicate andintricate balance of these properties must be maintained in order

Nonwovens ContainingImmobilized SuperabsorbentPolymer ParticlesBy Darryl L. Whitmore, BASF Corp., Portsmouth, Virginia

ORIGINAL PAPER/PEER-REVIEWED

35 INJ Fall 2003

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to avoid saturation of the web structure during the monomercoating process. Saturation, where droplets of monomer solutioncoalesce on the web, leads to agglomeration and film formation.Figure 3 shows the functional relationship between SAP basisweight and weight percent loading for 55, 80 and 100 gsm sub-strates at a constant substrate density of 0.018 g/cc. Once the sat-uration zone is reached for a given substrate, film and agglomer-ate formation begins to occur and negatively impacts the flexibil-ity of the structure. In addition, this phenomenon alsoadversely affects the absorbency rate of the applied SAP.

The monomer coating process is also critical to SAPdroplet formation. For spray application, the degree ofatomization is controlled by: (1) the viscosity of thecoating (the higher the solution viscosity with the highshear rate encountered going through the nozzle ori-fice, the larger the particle size), (2) atomization gaspressure (higher gas pressure giving smaller particlesize), (3) diameter of the nozzle orifice (smaller orificegiving smaller particles), (4) pressure forcing the coat-ing through the orifice (higher pressure leading tosmaller particle size), and (5) surface tension (lowersurface tension yielding smaller particle size) [5]. Bymaintaining a critical balance among all of these para-meters, the SAP particle size on the web can be con-trolled. Both fiber type and finish also play a crucialroll in droplet formation. As shown in Figure 4,hydrophobic fibers and finishes promote more spheri-cal droplet formation, while hydrophilic types tend topromote film formation of the applied hydrophilicmonomer mixture.

Other factors to consider when choosing a nonwoven for in-situ SAP polymerization include flexibility, softness, compress-ibility, resilience and elasticity. For hygiene applications, the finalproduct must, of course, remain soft and flexible even though itis generally compressed to produce a thin structure. Resiliencyand elasticity are important aspects of the nonwoven in that thesubstrate must be able to expand as the SAP particles begin toswell. Any impedance to the swelling of the SAP particles in the

36 INJ Fall 2003

Figure 1PAD FORMATION — CONVENTIONAL VS. IN SITU

Figure 2IN-SITU SAP POLYMERIZATION ON

A FIBROUS SUBSTRATE

SAP Particle (<400 mm)

SAP Bond Point

Web Fiber

Pore Volume

Highly permeable structures with good absorption properties thatsimplify the manufacturing process for superabsorbents and diapers

xyz directions may prohibit the SAP from achieving its fullabsorptive capacity.

ApplicationsOne unique property of in-situ SAP structures is their ability to

rapidly generate a large, mechanically stabilized pore volume,which makes the structure become highly permeable [6]. In-situmaterials provide such a mechanism through the swelling of theimmobilized SAP particles following exposure to fluid. Both thedegree and rate of expansion may be optimized by adjustingsuch parameters as: the degree of loading of SAP particles on theweb, the SAP particle size, the degree of neutralization of the SAPparticles and their respective crosslink density. The degree ofexpansion is an indication of the generated pore volume avail-able for fluid uptake (i.e. larger volume correlates with better per-formance) and by increasing the speed at which the pore volumeis generated, the likelihood of leakage during rapid fluid appli-cation is diminished.

The degree and rate of expansion of in-situ materials may bedetermined by measuring the free swell expansion volume(FSEV) [6]. The FSEV is determined by measuring the height(thickness) change, in millimeters, of a compressed, 6.0 cm diam-eter circle of the test material during hydration using a single 20ml dose of 0.9% saline under an applied load of 0.6 KPa. The

thickness of the sample as a func-tion of time is measured with theaid of a linear variable differentialtransducer (LVDT) interfaced to acomputer (additional details aboutthis method may be found in ref-erence 6). The results obtained foran in-situ material, containing 100gsm of SAP applied to a 55 gsm,thermally bonded high-loft non-woven, is shown in Figure 5a. Asimilar test, expansion volumeunder load (EVUL), was also con-ducted on the same sample usingan applied load of 3.0 KPa and isshown in Figure 5b. As shown inFigures 5a and 5b, the in-situ mate-rial expands quite rapidly oncehydrated, even under an appliedload of 3.0 KPa.

Following both the FSEV and EVUL tests, the hydrated in-situsample was removed from the sample cell and the free fluid con-tained within the void spaces of the material was removed byblotting the sample between two stacks of filter paper under anapplied load of 3.0 KPa for two minutes. The resulting weightloss allowed an estimation of the void volume of the sample. Thecorresponding gel volume, measured by the weight change ofthe dewatered sample, gives the amount of the fluid containedwithin the in-situ SAP particles on the web. The results of thisdetermination are shown in Table I. As can be seen from the data,the in-situ material generates a substantial pore volume com-pared to the uncoated high loft nonwoven substrate and the SAPparticles provide fluid storage.

Since in-situ materials swell rapidly and generate a large openpore volume, they lack sufficient capillary pressure to effectivelywick and distribute fluids. This limitation is effectively overcomethrough the formation of laminate structures with cellulosicmaterials. Suitable laminates may be prepared by either air lay-ing fluff pulp directly on the in-situ web or by simply laminatingthe webs with an air-laid, fluff-pulp roll good as shown in Figure6. The resulting laminate may then be either layered or folded asshown in Figure 7 to produce a structure with the desired basisweight and SAP loading.

A four layer, in-situ and fluff-pulp laminate structure is shownin Figure 8. A comparison of the inclined wicking performanceagainst two commercial adult diapers is shown in Figure 9.Inclined wicking is determined by measuring the saline uptakeof the sample up a 30 degree incline. One end of the sample isimmersed in a constant-level reservoir containing 0.9% saline.Fluid uptake is measured by an electronic balance interfaced to acomputer. As shown in Figure 9, the wicking performance of thein-situ laminate structure is comparable to that of the two com-mercial adult diapers. Experimental diapers were prepared byremoving the cores from both the low SAP (10 wt%) and highSAP (40 wt%) diapers and replacing them with the four layer, in-situ laminate. The results obtained from rewet testing are shownin Figures 10a and 10b. As shown in Figure 10a, a significant

37 INJ Fall 2003

Figure 3FUNCTIONAL RELATIONSHIP BETWEEN SAP BASIS WTS. AND WT.

% LOADING

Figure 4EFFECT OF FIBER FINISH ON SAP

DROPLET FORMATION

600

500

700

SAP

(gsm

)

improvement in rewet performance was observed for the low-SAP diaper containing the in-situ laminated core, while compa-rable performance was obtained for the modified high-SAP dia-per. A dramatic improvement in strikethrough performance wasobserved (Figure 10b) for both diapers containing the in-situcore, demonstrating the superior fluid acquisition properties ofin-situ materials.

Considering the acquisition and rewet data, we canaddress a common, undesirable trend seen among manycommercially available diapers on the market today: thefact that acquisition times tend to increase with succes-sive doses of fluid during rewet testing. With conven-tional fluff-based absorbent structures, the cellulosicfibers can lose resiliency and collapse when wetted. As aresult, the liquid uptake rate of the wetted structuresmay become too low to adequately accommodate subse-quent fluid insults. In these conventional structures,where absorbent gelling particles are incorporatedbetween the fibers to hold them apart, the gelling parti-cles swell and do not release the fluid. Swelling of theparticles can then diminish the void volume of theabsorbent structure and reduce the ability of the struc-ture to rapidly uptake fluid. The degree to which the

swelling of the absorbent gelling particle negativelyimpacts the rate of fluid uptake is dependent upon anumber of factors, such as the concentration of superab-sorbent used in the absorbent core, the degree of cross-linking in the polymer, the uniformity of the distributionof SAP within the structure, the particle size distributionof the powder and the hydrophobicity of the particles.While many of these factors are difficult to control inconventional cores, each of these factors is easily con-trolled with an in-situ core structure and may be opti-mized to achieve the desired performance properties fora given absorbent article.

In many current, commercially manufacturedabsorbent products, a considerable amount of pressureis applied during manufacture to produce an "ultrathin"product. This compression decreases the interstitial porevolume of the structure, resulting in slower fluid uptake.Currently, a section of a high-loft nonwoven is inserted

between the topsheet and the absorbent core to compen-sate for this effect and aid in the uptake of fluid. Keyproperties of this type of acquisition layer are the wet

and/or dry modulus of the constituent fibers, the hydrophilicityof the fibers and the resiliency of the fabric structure [7]. Suchproperties contribute to the acquisition layer’s ability to stayopen under load, maintain void volume, resist wet collapse,enhance the desorption properties of the fabric, and preservevoid volume capacity after successive applications of fluid.Acquisition materials used in current products have no real

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Figure 5aFREE SWELL EXPANSION VOLUME (0.6 KPa)

Figure 5bEXPANSION VOLUME UNDER LOAD (3.0 KPa)

TABLE 1SUMMARY OF RESULTS OF FSEV AND EVUL TESTS FOR IN-SITU SAP NONWOVEN

Initial Final Expansion Gel CapillaryTest height (mm) Height (mm) Volume (ml) Volume (ml) Volume (ml)FSEV 0.72 3.5 9.8 3.1 6.2EVUL 0.61 2.5 7.1 3.4 3.1FSEV Uncoated Web 0.45 0.45 1.3 0.03 1.1

mechanism to reopen after being compressed other than thememory effects preserved in the fibers themselves. In-situ corestructures possess their own built-in acquisition capability pro-vided by their rapid generation of a large, mechanically stabi-lized pore volume.

In-situ SAP polymerization also offers the potential to produceprofiled absorbent structures in which each individual web sur-face has been spray coated with superabsorbent particles of dif-ferent performance characteristics that have been optimized toperform a specific function, such as acquisition, fluid distributionor fluid storage. An example of such an optimized structure

would be a web structure inwhich one coated web surface hasbeen optimized for fluid acquisi-tion and the other for fluid stor-age. Webs prepared in this man-ner may be used either alone in anabsorbent article to provide thesole means of fluid acquisitionand storage or in combinationwith conventional fluff and SAPcores to provide improved fluiduptake and increased capacity inselected zones within the article.The performance characteristics ofspray-coated webs are dependenton a number of different factors,such as the crosslink density anddegree of loading of the formedsuperabsorbent polymer as wellas the density and basis weight ofthe substrate itself.Typically, acquisition zones arehighly crosslinked to both mini-mize fluid retention and increaseswelling speed, while storagezones are optimized for capacity.It is also possible to produce SAPgradients in both the xy plane andz direction of the nonwoven.Such possibilities do not easilyexist with conventional absorbentcore forming technologies.It is well known that an absorbenthygiene product capable of lower-ing skin pH within the range of4.0 to 5.5 is beneficial in prevent-ing, or at least reducing, the inci-dence of diaper rash [8, 9, 10].Attempts to lower the pH of dia-pered skin through the addition ofvarious acidic pH control agentssuffer from a number of draw-backs including: decreasing theabsorptive capacity of theabsorbent core, safety and comfort

factors associated with leaching of the pH-control materials fromthe article and processing problems associated with the place-ment and distribution of the acidic material within the absorbentcore. Through simple control of the degree of neutralization ofthe applied acrylic monomer solution, in-situ SAP technologyoffers a convenient means to lower the pH both within the inte-rior of the diaper as well as the surface without the difficultiesmentioned above [6].

ConclusionsIn-situ, SAP-containing nonwovens have been shown to pos-

39 INJ Fall 2003

Figure 6IN-SITU LAMINATED CORE STRUCTURES

Figure 7IN-SITU LAMINATED CORE STRUCTURES

sess a number of unique properties for application in the per-sonal hygiene industry, such as improved fluid acquisition, per-meability, compressibility and pH control. These materials alsoprovide a more homogeneous SAP distribution, eliminate theneed for SAP powder handling and have superior wet integritycompared to conventional fluff pulp and SAP air-laid structures.This technology also offers some unique opportunities fordesigning and manufacturing profiled absorbent articles withspecific zones tailored to perform specific functions.

References1. Pieniak, et al., “Superthin absorbent product,” U.S. Patent

4,573,988, Mar. 4, 19862. Erdman, et al., “Stable disposable absorbent structure,” U.S.

Patent 4,676,784, Jun. 30, 19873. Pieniak, et al., “Superthin absorbent product,” U.S. Patent

4,537,590, Aug. 27, 19854. Pieniak, et al., “Method of forming a superthin absorbent

product,” U.S. Patent 4,540,454, Sep. 10, 19855. Z. Wicks, Jr., F. Jones, S. Pappas, Organic Coatings (2nd ed.),

pg. 420, John Wiley & Sons, New York, NY (1999)6. Whitmore, D., Engelhardt, F., “Absorbent article and process

for preparing an absorbent article,” U.S. Patent 6,417,425 B1, Jul.9, 2002

7. Latimer et al., “Absorbent structure having improved fluidsurge management and product incorporating same,” U.S.Patent 5,364,382, Nov. 15, 1994

8. Zimmerer, “Disposable absorbent articles,” U.S. Patent4,657,537, Apr. 14, 1987

9. Blaney, “Disposable diaper containing ammonia inhibitor,”U.S. Patent 3,964,486, Jun. 22, 1976

10. Alonso, et al., “Composition for treatment and preventionof malodorous generating skin conditions,” U.S. Patent4,382,919, May 10, 1983 — INJ

40 INJ Fall 2003

Figure 8IN-SITU WEB LAMINATE FOR DIAPER

CORE CONSTRUCTION

Figure 9INCLINED WICKING COMPARISON OF IN-

SITU COMPOSITE AND ADULT BRIEF CORES

Figure 10aREWET COMPARISON OF ADULT MEDIUM

BRIEFS AND IN-SITU SAP WEB CORE

Figure 10bSTRIKETHROUGH COMPARISON OF ADULT

MEDIUM BRIEFS AND IN-SITU SAP WEB CORE

STR

IKET

HR

OU

GH

(SE

C)